Modern genetics can be seen as the result of the integration of three lines of investigation: breeding tests, cytology, and biochemistry. Classical genetics could address the question of how the gene was transmitted, but it could not explain how genes work. Indeed, some geneticists argued that it was impossible to imagine that any particle or molecule could constitute the genetic material. Others were sure that the gene must be a chemical entity with a specific, defined molecular arrangement. Once scientists were able to identify DNA as the genetic material and a model for the three-dimensional structure of DNA was proposed, molecular biologists were able to investigate the biochemical basis of gene structure and function.

The correlation between chromosomal structure and measurements of linkage became well established in the 1930s for several plant and animal species. During this time period...